A thermoelectric conversion material is a material which can interconvert thermal and electric energies, and constitutes a thermoelectric converter which is used as a thermoelectric cooling element or a thermoelectric power generating element. Thermoelectric conversion material is used for thermoelectric conversion using the Seebeck effect. Thermoelectric converting performance is represented by Formula (1) which is called the performance index ZT.ZT=α2σT/κ  (1)(wherein, α represents Seebeck coefficient, σ represents the electrical conductivity, κ represents the thermal conductivity, and T represents the measured temperature.)
It is apparent, according to Formula (1), that in order to improve the thermoelectric converting performances of a thermoelectric conversion material, Seebeck coefficient α and electrical conductivity σ of the material are increased, and thermal conductivity κ of the material is decreased. Japanese Unexamined Patent Publication 10-242535 describes adding fine particles (inactive fine particles) which do not react with a base material of a thermoelectric conversion material to particles of a starting material of the thermoelectric conversion material in order to decrease thermal conductivity κ of the material. Thereby, the Inactive fine particles can scatter phonons, which are the major factor of the thermal conduction in a thermoelectric conversion material, to decrease thermal conductivity κ.
However, in a conventional conversion material in which the inactive fine particles are unevenly distributed, the inactive fine particles, which provide the scattering effect of the phonons, have a large adverse influence on the other physical properties, such as electrical resistivity, due to the uneven distribution thereof, thus an increase in the performance of the thermoelectric conversion materials is inhibited. In order to solve this problem, Japan Unexamined Patent Publication 2000-261047 discloses a thermoelectric conversion material comprising metal or alloy particles having a submicron particle size to hundreds of microns order dispersed in the particles of the thermoelectric conversion material.
According to the disclosure of above Japan Unexamined Patent Publication 2000-261047, electrical conductivity is improved by compounding a metal as a dispersant, and thermal conductivity is also reduced because a thermal barrier is formed at a border between the metal powders and the matrix of the thermoelectric conversion material.
However, the metals or alloys as the dispersant have a particle size of submicron to a few hundreds micron order and therefore, it is impossible to disperse the metals or alloys into the thermoelectric conversion material in the nanomicron order. Further, grain-growth of the metals or alloys may occur during sintering, as a result, the distance between the metals is larger than the mean free path of the phonons of the thermoelectric conversion material, and consequently thermal conductivity cannot be sufficiently reduced.
Since the carrier (an electron or electron hole) can carry both heat and electricity, electrical conductivity σ and thermal conductivity κ are proportional. Additionally, it is known that electrical conductivity σ and Seebeck coefficient α are inversely proportional. Therefore, if electrical conductivity σ is increased, thermal conductivity κ is increased and Seebeck coefficient α is decreased accordingly. Furthermore, as the effective mass and mobility are inversely proportional, the effective mass is decreased when the mobility is increased.
Therefore, the object of the present invention is to eliminate the drawbacks of the prior art stated above by providing a method for manufacturing a thermoelectric converter having a good performance index.